Honestly not sure why, but here’s a list of suggestions for try:
check your Subnet masks are correct on all devices
hopefully your laptop has an Ethernet port, if so try plugging into both routers and check if you can connect to the ip camera? If still not, can you at least ping it?
it may be something to do with arp cache, either in the ip camera or in the routers. As a test you could try changing the subnet for the first floor router + devices to something else. This would force your ground floor router to route the packets instead of switching them. You’d need to create some extra routes on both routers so they know where to forward the traffic tho
double check all devices in your home network side of openwrt devices are in the lan firewall zone , or you’ve opened access with rules
edit: forgot to mention, try sshing onto your first floor router and see if you can ping the ip camera from there too
Connect the two routers by the LAN ports. The upstairs router needs to be configured as a "dumb AP". Everything connected to it will bridge to downstairs which does the actual routing.
The ISP box and its LAN should be considered part of the Internet. Everything in the house goes on the LAN side of the downstairs Xiaomi. Turn off the WiFi in the ISP box and use only one of its Ethernet ports to connect to the WAN port of the downstairs router.
If your going to use the same subnet for both routers, then this setup unlikely to work - you’ll need to connect lan <-> lan ports between the routers.
If you use different subnets for the lan interface on each router then lan > wan will work, but without firewall rules on the first floor router (or disabling the firewall) you’ll probably find that nothing connected to the ground floor router will be able to reach stuff on the 1st floor router .